(rō'dē-əm) Symbol RhA rare, silvery-white metallic element that is hard, durable, and resistant to acids. It is used as a permanent plating for jewelry and is added to platinum to make high-temperature alloys. Atomic number 45; atomic weight 102.905; melting point 1,966°C; boiling point 3,727°C; specific gravity 12.41; valence 2, 3, 4, 5, 6. See Periodic Table.